1. Field of the Invention
The present invention relates to a selective catalytic reduction (SCR) device that is mounted in a vehicle, and more particularly to a system for controlling urea injection in the vehicle and a method thereof
2. Description of Related Art
A variety of post-processing devices are mounted in vehicles so as to eliminates harmful materials such as NOx, CO, THC, soot, particulate matter (PM), and so on that are included in exhaust gas so as to correspond to regulations for a North America diesel Tier2/BiN5 or regulations for EURO 6.
The post-processing devices includes a diesel oxidation catalyst (DOC) that is disposed at a downstream side of an engine, a catalyzed particulate filter (CPF) that filters particulate matter (PM), and an SCR device that reduces NOx through a reduction reaction.
The selective catalyst reduction system uses ammonia (NH3) as a reducing agent so as to purify NOx that is exhausted out of a diesel vehicle in a large quantity. The ammonia has good selectiveness to the NOx, and even though oxygen exists, the ammonia reacts with well the NOx.
A chemical reaction of the NOx with ammonia (NH3) is as follows:NO+NO2+2SACIDNH3→2N2+3H2O4NO+O2+4SACIDNH3→4N2+6H2O6NO2+8SACIDNH3→7N2+12H2O   Reaction Formula 1
A dosing injector injects a urea solution so as to purify the NOx through the SCR device, and the NH3 that is generated from the injected urea solution eliminates the NOx.
A mixer is disposed between the dosing injector and the SCR device, and it mixes the urea particles that are injected through the dosing injector and guides the flow direction of the urea particles.
Accordingly, the urea particles are uniformly mixed with the exhaust gas and the purification efficiency of the SCR device is improved.
In an urea injection control method that is applied in a conventional vehicle, if ignition-on before starting an engine is detected, a pump that is disposed inside a urea tank is operated to supply the dosing injector with the urea solution at a uniform pressure of about 5 bar and the dosing injector injects the urea solution.
The set position of an ignition key includes an “ignition-off” state, an “ignition-on” state, and a “starting state” that respectively correspond to the rotation position of the ignition key.
In this specification, an “engine-on” state designates that the engine is operating and an “engine-off” state designates that the engine is not running.
More specifically, the ignition-on state of the ignition key includes one state in which the engine is operating and another state in which the engine is not operating.
When the ignition key is rotated from the ignition-on (engine-off state) to the starting state, a starter motor operates to start the engine and then if the engine is started, a driver of the vehicle returns the ignition key from the starting state to the ignition-on state (engine-on state).
Also, if the ignition key is rotated from the ignition-on of the engine-on state to the ignition-off state, the engine stops operating to have the engine-off state.
If the ignition key is positioned to ignition-off by the driver during the operation of the engine, a reverting valve that is disposed at the pump is operated in order to prevent freezing and bursting of the urea supply line.
Accordingly, the urea solution is not supplied to the urea supply line from the pump and simultaneously the dosing injector is opened to inject the urea solution of the supply line into an exhaust pipe.
However, when the urea solution is injected into the exhaust pipe during the engine-off state in which the engine is not operating as stated above, solid urea crystals are repeatedly formed at a downstream side of the exhaust pipe as shown in FIG. 3 and FIG. 4.
The attached urea crystals can be eliminated at a temperature of higher than 600° C. that is achieved when the catalyzed particulate filter is regenerated and in a condition in which the flux of the exhaust gas is high, but the urea crystals are not substantially eliminated under normal driving conditions.
In addition, when the attached urea crystals are detached, the NH3 can be excessively discharged to the outside and thereby the quality of the exhaust gas can be deteriorated.
Particularly, the urea crystals that are attached on a guide fin of the mixer transform the shape thereof such that the performance of the mixer and the purification efficiency of the NOx are deteriorated.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.